Mark Shaw, age 14, of Cumberland, Md., for his question:
WHAT IS POLARIZED LIGHT?
Polarized light consists of light waves that have a simple, orderly arrangement. The waves of ordinary light are arranged in a complex, disorderly manner.
Ordinary light from the sun or a lamp in your house is composed of disorderly waves that vibrate in all directions perpendicular to the light beam. But polarized light consists of orderly waves that vibrate only in one direction.
Because of its orderly structure, polarized light can be used in ways that would be impossible with ordinary light. As an example, the internal physical structure of many transparent materials can be seen with the aid of polarized light. Light polarizers are powerful tools that are used in science, industry and everyday life.
To understand polarization, imagine a light beam as a train of electromagnetic waves. The electromagnetic forces making up these waves vibrate in a crosswise direction, perpendicular to the path of the beam.
You can get an image of what these waves look like by attaching a rope to a wall and shaking the other end. A train of waves will move along the length of the rope. Each part of the rope will vibrate not lengthwise, however, but crosswise. Waves that vibrate in this way are called transverse waves.
Ordinary light can be polarized by passing it through a special light polarizing filter. This filter allows only the waves that vibrate in one crosswise direction to pass through. The structure of the filter prevents the passage of light waves that vibrate in other crosswise directions.
In scientific terms, the polarization filter allows the components or parts of the light waves that vibrate in one vibration direction to pass through. The components of waves that vibrate in all other directions are held back.
The light that passes through the light polarizing filter is called polarized light.
There are many practical uses of polarized light in addition to sunglasses and photography. Scientists have suggested that polarized glass be used for car headlights and windshields to prevent driving glare from lights of approaching cars.
Scientists can study the structure of many transparent materials with the aid of crossed polarizing filters. Microscopes equipped with polarizers show many colorless crystals and biological specimens in brilliant color.
A polariscope, an instrument equipped with polarizers, is used to find strains or weak spots in glass objects such as sunglasses and laboratory glassware. Chemists can tell the type and amount of sugar in a solution by using a saccharimeter, a type of polariscope.
Special polarizing filters that produce circularly polarized light are also used on radarscopes to trap unwanted reflections.
The most widely used light polarizers consist of thin plastic sheets. A typical plastic sheet contains millions of long, slender, carefully aligned chains of iodine molecules.